A conventional technology used for manufacturing an electro-acoustic transducer is described with reference to FIG. 5 through FIG. 7.
FIG. 5 shows a cross sectional side view of a sounder for use in a mobile communication apparatus. FIG. 6 is a cross sectional view of an enameled wire for forming a coil, which is a key portion of the sounder. FIG. 7 is a perspective view used to describe how the enameled wire is soldered by virtue of laser irradiation.
Referring to FIG. 5, an enameled wire 1 is wound to form a coil 1a disposed on a plate 3 having a central pole 2. A terminal 4 is resin-molded in a bottom case 4a with at least a soldering portion of the terminal exposed outside of the bottom case. The terminal is provided at the exposed portion with a land for connection with the enameled wire 1. A ring-shaped magnet 5 is fixed on the plate 3. Above the magnet 5 is a diaphragm 6, which is disposed with a certain clearance from the magnet 5. A small piece of magnetic material is attached on the diaphragm 6 at a center of the diaphragm. An upper case 7 is provided with a sound radiation hole 8.
Enameled wire 1 constituting the coil 1a is described with reference to FIG. 6. The enameled wire wire 1 includes a core wire 1b of copper or an copper alloy, which is covered around an outer surface thereof with an insulating film 1c, and a hot-melt layer 1d covers the insulating film 1c. 
During winding of the enameled wire around the central pole 2 for forming the coil 1a, the hot-melt layer 1d is softened or melted by hot air blown thereto, or by another manner. When heating with hot air is stopped, the hot-melt layer 1d solidifies again, and as a result the enameled wire 1 stays in a coiled shape.
An end part of enameled wire 1 forming the coil 1a is soldered to the terminal 4 after the insulating film 1c is stripped.
A generally practiced process of soldering is either; stripping the insulating film 1c using a CO2 laser or the like before soldering, and then soldering the wire; or performing stripping and the soldering at the same time using a laser beam.
A method where stripping the insulating film 1c and soldering of the wire are performed simultaneously with a laser beam is described below with reference to FIG. 7.
A conventional soldering apparatus consists mainly of a laser oscillator 10, an optical fiber 11, a projection lens 12 and a solder string 13. A laser beam from the laser oscillator 10 is led via the optical fiber 11 to the projection lens 12 to be projected to an exterior thereof. The laser beam is absorbed by the land of terminal 4 and the enameled wire 1, and the beam is converted into thermal energy. When the land of terminal 4 is heated to reach a solder melting temperature, the solder string 13 is delivered thereto. Heat energy provided for soldering removes the hot-melt layer 1d and the insulating film 1c by melting, evaporation or sublimation, and solders the enameled wire 1 to the land.
In the above-described method of soldering, where an insulating film and hot-melt layer are stripped away prior to soldering, cost of bonding tends to increase because of increased process steps required for soldering. In addition, a CO2 laser used for stripping the insulating film 1c readily oxidizes the land, and as a result, a wetting property of the land against solder deteriorates.
In a method where stripping of insulating film 1c and soldering of wire 1 are conducted simultaneously by laser beam irradiation, since the normally-used core wire 1b of enameled wire 1 is as fine as 0.1 mm in diameter, the enameled wire 1 easily breaks by a slightly increased output of a laser beam which leads to an excessive generation of heat energy. On the other hand, if the heat energy is insufficient, the insulating film 1c is stripped imperfectly, which tends to cause soldering troubles such as a tunneling solder, or the like.
As described in the foregoing, a conventional soldering method using a laser beam has a drawback in that it requires a precise process control.
The present invention addresses the above problems and aims to provide an enameled wire with which a time needed for laser-soldering a wire on a soldering land can be shortened, and a rate of occurrence of tunneling solder and other soldering troubles can be decreased. The present invention also includes a method of soldering an enameled wire, as well as a soldering apparatus using the enameled wire of the present invention.